1. Field of the Invention
The present invention relates to method and apparatus for automatically adjusting white balance and, in particular, to method and apparatus for automatically adjusting white balance using an internal light measurement system.
2. Description of the Related Art
Conventionally, an automatic white balance adjusting method includes an external light measurement system and an internal light measurement system. In the external measurement system, a color temperature is measured by use of an external sensor and a control signal obtained from the measured color temperature is used to control the gains of red and blue signals, which is generally referred to as an open loop control. In this system, if the color temperature of the light that strikes on an object is different from the color temperature of the light incident upon the external sensor, then an error is produced to thereby provide a unnatural color tone. For example, when an exterior is photographed through a window from an illuminated room, since the interior light having a low color temperature strikes the external sensor, a control to make the color tone blue is effected so that the resultant image has a strong blue tone.
On the other hand, in the internal measurement system, since the light striking the object is used, a direct decision can be made and thus a suitable color tone can be obtained.
Now, description will be given here of the automatic white balance adjusting method according to the abovementioned conventional internal measurement system in connection with FIG. 15. The light entering through a lens 10 is photoelectrically separated by an image pickup element 12 into red, green and blue signals (E.sub.R, E.sub.G, E.sub.B). These signals (E.sub.R, E.sub.G, E.sub.B) are respectively input through amplifiers 14, 16, 18 to a matrix circuit 20, where they are converted into a brightness signal E.sub.Y and color difference signals (E.sub.R -E.sub.Y), (E.sub.B -E.sub.Y) and are then input into an encoder 22. In the encoder 22, the color difference signals are DC/AC two-phase modulated by means of 3.58 MHz signals and are added with the brightness signal E.sub.Y, and then are output as a video signal in the NTSC system.
On the other hand, the color difference signals (E.sub.R -E.sub.Y) and (E.sub.B -E.sub.Y) are respectively input to integrating circuits 24 and 26. In these circuits, the color signals on the entire screen are integrated and the average value of the integrations is then input to the respective negative inputs of differential amplifiers 28 and 30. To the respective positive inputs of the differential amplifiers 28 and 30, there is input a reference level from reference level setting means 32 and 34 which is the average value of the color difference signals (E.sub.R -E.sub.Y) and (E.sub.B -E.sub.Y) corresponding to the time when the average value of the colors o the entire screen indicates a grey color. The differential amplifier 28 amplifies the difference between the above-mentioned two input signals and then outputs to the amplifier 14 a gain control signal R.sub.CONT which is obtained from the difference amplification, thereby controlling the gain of a primary color signal E.sub.R which is amplified by the amplifier 14. Also, the differential amplifier 30 outputs to the amplifier 18 a gain control signal B.sub.CONT which is obtained by amplifying the difference between the above-mentioned two input signals, thereby controlling the gain of a primary color E.sub.B which is amplified by the amplifier 18. As a result, the primary color signals E.sub.R, E.sub.B can be controlled so that the average
value of the color difference signals (E.sub.R -E.sub.Y), (E.sub.B -E.sub.Y) on the entire screen coincides with the reference level.
In other words, the above-mentioned automatic white balance adjusting method is a control system based on a generally known fact that, when colors on the entire screen are average, a grey color is obtained (even when a general object is photographed, the color average on the entire screen provides the same result that is obtained when a white object is photographed.)
However, all objects cannot satisfy the above-mentioned supposition. An object with a blue sky and a blue sea for a background, or an object with a red wall for a background is an example of such objects. In this case, the integration (averaging) of colors on the entire screen results in a blue or red color, not a grey color. If the above-mentioned white balance adjustment is effected on such an object, then both blue and red colors turn into a grey color. This is an erroneous control which is generally referred to as a color failure.
Also, in the above-mentioned automatic white balance adjusting method, even when the color temperature with respect to the object is varied, the color of the object is adjusted to be the same color. This is truly preferable in that the thus adjusted color is coincident with a so called memory color of a human being (for example, even when a white object is slightly colored by illumination, the object is recognized as not colored but white); but, since the above-mentioned white balance adjustment is made at any color temperatures, when the color temperature is beyond a predetermined range, the resultant color provides an unnatural tone which is quite alien to the human sensitivity.
Further, for example, if while the white balance adjustment is being made on a reddish object the object itself is turned white suddenly, then the gain control signal R.sub.CONT is greatly varied, with the result that the amplifier 14, which is adapted to control the gain by means of the gain control signal R.sub.CONT, controls the gain of the signal E.sub.R more than necessary (in the direction of a complementary color of a red color), thereby causing vibration (hunting) until the control signal settles down to an actual control value.